Jaundice due to unconjugated hyperbilirubinemia in the newborn can lead to serious physiologic changes including cerebral palsy and death. The simplest, now most prevalent but least well understood treatment for this jaundiced condition is phototherapy. During phototherapy unconjugated bilirubin (USB) interacts with light and is both excreted as UCB and degraded to water-soluble products. The jaundiced condition is thus alleviated until the infant can begin to excrete its bilirubin. We have a program underway to investigate the interaction of bilirubin and light and have generated considerable new information. We are examining the mechanism of UCB excretion during phototherapy via Z yields E photoisomerism mechanism, which significantly alters the UCB aqueous solubility. We are also investigating bilirubin photochemistry in the presence and absence of oxygen and under a variety of solvent or phase conditions which mimic the in vivo situation. Thus, we are especially interested, inter alia, in excitation wavelength dependence on the course of the reactions, and the quantum yields for disappearance of UCB, the nature of the influence of albumin and micellar behavior.